1. Field of the Invention
The present invention relates to a sintered polycrystalline diamond composite for use in rock drilling, machining of wear resistant metals, and other operations which require the high abrasion resistance or wear resistance of a diamond surface. Specifically, this invention relates to such bodies that include a polycrystalline diamond layer attached to a cemented metal carbide substrate via processing at ultrahigh pressures and temperatures.
2. Description of the Art
Composite polycrystalline diamond compacts or PCD have been used for industrial applications including rock drilling and metal machining for many years. One of the factors limiting the success of PCD is the strength of the bond between the polycrystalline diamond layer and the sintered metal carbide substrate. For example, analyses of the failure mode for drill bits used for deep hole rock drilling show that in approximately thirty-three percent of the cases, bit failure or wear is caused by delamination of the diamond from the metal carbide substrate.
Furthermore, when a precemented carbide mass is relied on to increase the impact resistance of PCD, the diamond layer is preferably relatively thin so that the diamond is never too far from its support. This restriction on the thickness of the diamond layer naturally limits both the life expectancy of the composite compact in use as well as the designs for PCD diamond tools.
Yet another problem that has limited the thickness of the diamond layer in composite compacts is caused by the problem of "bridging". Bridging refers to the phenomenon that occurs when a fine powder is pressed from multiple directions. It is observed that the individual particles in a powder being pressed tend to stack up and form arches or "bridges" that block the full amount of pressure so that the pressure often does not reach the center of the powder being pressed.
For optimal abrasion resistance of the compact product, very fine crystals of the abrasive are typically used, generally in particle size of less than 10 microns and preferably less than 5 microns. The fine abrasive crystals are crushed further under the high pressures applied during the compaction process resulting in a packing density of around 1.5 grams/cc increasing to greater than 2.5 grams/cc by crystal fracturing. The resulting abrasive mass is very dense and offers resistance to the catalyst metal or catalyst metal and carbide from sweeping through the crystal interstices. In practice, this resistance to sweep through by the dense, fractured abrasive crystals leads to soft spots of non-bonded abrasive. These soft spots are especially prevalent when the layer of abrasive crystals exceeds about 1 mm in thickness. Coarser abrasive crystals offer channels in the compacted mass that are less torturous for the bonding metal to sweep through; however, abrasion resistance considerations usually preclude the use of such coarse crystals as starting materials for the compact.
One of the solutions to these problems is proposed in the teaching of U.S. Pat. No. 4,604,106. This patent utilizes one or more transitional layers incorporating powdered mixtures with various percentages of diamond, tungsten carbide, and cobalt to distribute the stress caused by the difference in thermal expansion over a larger area. A problem with this solution is that the cobalt cemented carbide in the mixture weakens that portion of the diamond layer because less diamond-to-diamond direct bonding occurs as a result of the carbide second phase.
Other patents have discussed using grooved substrates in order to both increase the thickness of the diamond layer at certain locations and to increase the bond strength between the diamond layer and the substrate. For example, U.S. Pat. No. 4,784,023 teaches the grooving of polycrystalline diamond substrates; but does not teach the use of patterned substrates designed to uniformly reduce the stress between the polycrystalline diamond layer and the substrate support layer. In fact, this patent specifically mentions the use of undercut or dovetail portions of substrate grooves, which contributes to increased localized stress. FIG. 1 shows the region of highly concentrated stress that results from fabricating polycrystalline diamond composites with substrates that are grooved in a dovetail manner. Instead of reducing the stress between the polycrystalline diamond layer and the metallic substrate, the use of dovetail grooving actually makes the situation much worse. This is because the larger volume of metal at the top of the ridge will expand and contract during heating cycles to a greater extent than the polycrystalline diamond, forcing the composite to fracture at locations 1 and 2 shown in FIG. 1.
The disadvantage of using relatively few parallel grooves with planar side walls is that the stress again becomes concentrated along the top and, more importantly, the base of each groove and results in significant cracking of the metallic substrate along the edges 3 of the bottom of the groove as shown in FIG. 2. This cracking significantly weakens the substrate whose main purpose is to provide mechanical strength to the thin polycrystalline diamond layer. As a result, construction of a polycrystalline diamond cutter following the teachings provided by U.S. Pat. No. 4,784,023 is not suitable for cutting application where repeated high impact forces are encountered, such as in percussive drilling, nor in applications where extreme thermal shock is a consideration.
Other configurations have been proposed in order to overcome problems of stress in the compact due to the mismatch in thermal expansion between the diamond layer and the tungsten carbide substrate. For example, U.S. Pat. No. 5,351,772 describes the use of radially extending raised lands on one side of the tungsten carbide substrate area on which a polycrystalline diamond table is formed and bonded.
U.S. Pat. No. 5,011,515 describes a substrate with a surface topography formed by irregularities having non-planar side walls such that the concentration of substrate material continuously and gradually decreases at deeper penetrations into the diamond layer. U.S. Pat. No. 5,379,854 describes a substrate with a hemispherical interface between the diamond layer and the substrate, the hemispherical interface containing ridges that penetrate into the diamond layer. U.S. Pat. No. 5,355,969 describes an interface between the substrate and polycrystalline layer defined by a surface topography with radially-spaced-apart protuberances and depressions.
All of the above proposals show a diamond layer of varying thickness relative to the surface of the tungsten carbide substrate support. Thus, in areas where the diamond layer is thicker, the amount of cobalt available is less than in those areas where the diamond layer is thin. This results in a non-uniformly sintered diamond layer that substantially weakens the compact. Even when cobalt powder is premixed with the diamond prior to subjecting the compact to high pressure-high temperature conditions, the presence of cobalt in a substrate with a textured surface produces areas of varying concentration of cobalt within the diamond layer during the sintering process and causes soft spots or poorly sintered areas within the diamond layer.
U.S. Pat. No. 4,311,490 teaches the use of coarse diamond particles next to the tungsten support with a layer of finer diamond particles placed on top as the exposed cutting surface. This is reported to reduce the occurrence of soft spots or poorly sintered areas in the diamond table since the coarser particles have larger channels between them making it easier for cobalt to sweep through the diamond nearest the tungsten carbide substrate, thus allowing thicker diamond layers to be sintered. For rock drilling applications, however, it has been found that although finer diamond results in higher abrasion resistance, it also results in significantly less impact resistance. The lower impact resistance produces compact cutter failure by way of fracturing and spalling of the diamond layer from the tungsten carbide support substrate.
Thus, two problems remain: one of producing a compact with the advantages of using a substrate with a non-planar interface without the drawback of soft spots or otherwise poor, non-uniformly sintered areas and second, maintaining a higher abrasion resistant compact for rock drilling without loss of impact resistance.